1. Field of the Invention
The present invention relates to a driving method for a piezoelectric vibrator, and more particularly, to a driving method for a piezoelectric vibrator for use in an ultrasonic motor to be used in autofocus driving of a single lens reflex camera, for use in a dust removing device for removing dust adhering onto an optical element of a single lens reflex camera, or for other similar uses.
2. Description of the Related Art
Conventional examples of a device using a piezoelectric vibrator constituted by a piezoelectric element are an ultrasonic washer (see Japanese Patent Application Laid-Open No. H11-253900) and a dust removing device (see US 2009/207493 A1), or the like.
In those devices, the piezoelectric vibrator is caused to vibrate by applying the piezoelectric element with an alternating voltage having a frequency close to the resonance frequency of mechanical vibration of the piezoelectric vibrator, and the generated vibration is caused to act on another component, to thereby attain the feature of the device.
As described in Japanese Patent Application Laid-Open No. H11-253900, it is known that a piezoelectric vibrator suffers from variations in characteristics, such as the deterioration, after long-term use.
In the ultrasonic washer described in Japanese Patent Application Laid-Open No. H11-253900, however, a sound pressure sensor is provided for detecting deterioration of an ultrasonic vibrator, thus causing a problem in that the size of the device increases to increase manufacturing cost.
The piezoelectric vibrator in the dust removing device of US 2009/207493 A1 includes, in addition to a driving piezoelectric element, a sensor which is constituted by a vibration detection piezoelectric element formed of the same piezoelectric material as that of the driving piezoelectric element, thus being capable of preventing the size of the device and manufacturing cost from being increased much.
Detecting the deterioration of the ultrasonic vibrator by using the vibration detection piezoelectric element formed of the same material as that of the driving piezoelectric element, however, causes the following problem. According to the dust removing device, in the case where there is no change (or a small change) in an output voltage per unit amplitude of the vibration detection piezoelectric element between before and after the deterioration of the piezoelectric vibrator, the same vibration amplitude can be obtained as follows.
In other words, even after the deterioration, the same vibration amplitude as before the deterioration can be obtained by adjusting an input so that the output of the deteriorated vibration detection piezoelectric element may be equal to the output of the vibration detection piezoelectric element before the deterioration.
It is conceivable, however, that the variations in characteristics of the piezoelectric vibrator are caused not only by the increase in mechanical resistance of the piezoelectric vibrator by long-term use but also by deterioration in piezoelectric characteristic of the piezoelectric element.
Depending on the factor of the variations in characteristics of the piezoelectric vibrator, the above-mentioned conventional adjusting method cannot always adjust the input with high accuracy.